Do you remember the Scarecrow from the story Wizard of Oz? He was clever without a brain, and so are plants. Plants are modular organisms, capable of learning, remembering, and making decisions. In their own pace of life, plants take the most of their environment to make intelligent choices.

A not-so-quiet life
Historically, people used to see plants as passive automatons. The term ‘vegetative’ used to address humans with cerebral death is a vivid example of this mindset. Indeed, plants are sessile organisms. In their evolutionary path, they adopted survival strategies slower and more economical than those adopted by animals. Locomotion is useless for plants, as they don’t need to move to collect solar energy and produce their food in the photosynthesis process. However, they are not as quiet as they seem to be. Apparently passive at the first sight, plants are dynamic and show other types of movement, such as growth, decay, and change of state (e.g., flowering, fruiting).

Plant Intelligence
For plants, lacking a brain is a brilliant survival strategy, because herbivores could eat it at any moment. If they do not have a brain, where is their intelligence? Simple, it is everywhere. Instead of a central brain, the entire plant works as a diffuse brain composed of modules. Each module acts as a parallel processor and plays several roles, including the production of new modules. Therefore, if a plant loses some modules in a herbivore attack, the system can still work and repair its loss without compromising plant survival.

Plants are highly sensitive to their environment. They can provide flexible responses to environmental stimuli and even anticipate future problems. Intelligence in plants can be defined as an adaptive and variable behavior. An alternative definition of plant intelligence is the ability to process information from biotic and abiotic stimuli and make optimal decisions. Either way, plant intelligence is based on plant behavior to increase fitness, i.e., the number of viable seeds.

A typical plant behavior is root foraging for rich soil patches and water, when roots actively search the soil for the best sources of water and nutrients. Plants can also recognize related plants, which is very useful in reproduction. They undergo fast morphological alterations in response to environmental changes (e.g., water stress or shading) to maintain ideal internal conditions. There are also extreme examples of plant behavior, such as choice for suitable hosts as in the parasitic plant dodder (Cuscuta sp), and active light foraging movements as in the Seychelles stilt palm (Verschaffeltia splendida), in which the plant ‘walks’ in search for sunny patches.

Plant Neurobiology
Plant neurobiology is a branch of biology research that studies plant behavior. It aims to understand how plants perceive, process information from the environment, and respond to it, so they develop and reproduce optimally.

Plant Signaling
Signaling and memory are paramount in plant responses. Still in the early 20th century, the electrophysiologist, Sir Jagadish Chandra Bose reported electrical signaling between plant cells in coordinated responses to the environment. At that time, his findings were not well received by other scientists.

Many years later, Indrani Bose and Rajesh Karmakar, from the Department of Physics in Bose Institute, Kolkata, India proposed simple models of plant learning and memory based on calcium signaling system. Memory states correspond to steady state distributions of calcium ions. The scientists affirm that the flow of calcium wave is analogous to the transmission of electrical pulses in a neural network, capable of computing, learning, and remembering.

In addition to calcium signaling system, action potentials and voltage-gated channels are also found in plants. Action potentials act in the recognition of herbivorous attack, flower induction, and phototropism (tendency to grow towards or against light). Other plant functions, such as phloem transport, response to pollination, changes in respiration and photosynthesis rates, and plant defenses require electrical signals.

Plant learning and memory
Memory is the ability to access past experiences so that new responses incorporate information from the past. Plants have three types of memory: sensory, short-term, and long-term memory. The latter includes even a transgenerational memory, passed to the next generation at the cellular level, for example in a long-term maintenance of a gene expression pattern.

Scientists from the International Laboratory of Plant Neurobiology of the University of Florence, Italy tested the learning capacity and memory of the shy plant Mimosa pudica. This creeping herb folds its composed leaves when touched or shaken and re-opens them some minutes later. To test learning, the scientists submitted individual M. pudica to repeated falls at regular intervals. At first, the plant closed its leaves, but some time later, it learned the fall did not represent a threat and kept its leaves open. Next, to test plant memory, the scientists let the plants rest for some time and repeated the experiment. Thirty days later, M. pudica still maintained its leaves open when submitted to falls.

Other groups of scientists, from different research institutes, tested plant memory in several experiments that basically divided plants into two groups: experimental and control. The experimental group was exposed to a mild stress agent (e.g., low water supply, temperature, low concentration of salt or cadmium). Then, both groups were exposed to extreme stress conditions (drought, cold, high concentration of salt or cadmium). As a result, previously exposed plants showed higher survival rates. Hence, plant morphological changes function as long-term memory and influence subsequent behavior.

Talking Plants
Plants also communicate with other plants and even animals. They influence and receive influence from their neighbors both in competition and cooperation relationships.
It is widely known that plants compete for resources, such as space, water, light, and nutrients. What is not so widespread is that they also cooperate. One example of this positive interaction is the interspecies trade of nitrogen and carbon through a symbiotic relationship with fungi (ectomycorrhizae), which forms a subterranean communication network. Communication can also occur in the air. Some plants warn their neighbors about herbivorous attacks by releasing volatile chemical substances and attract pollinators with perfumes released from their flowers.

Intelligent plants don’t wish they had brains
In short, plants do not need brains to interpret complex aspects of their environment and change their behavior to optimize their fitness. Their development is not pre-determined, but responds dynamically to environmental resources available.

Bose, I. and Karmakar, R., 2003. Simple models of plant learning and memory. Physica Scripta, 2003(T106), p.9.
Brenner, E.D., Stahlberg, R., Mancuso, S., Vivanco, J., Baluška, F. and Van Volkenburgh, E., 2006. Plant neurobiology: an integrated view of plant signaling. Trends in plant science, 11(8), pp.413-419.
Marder, M., 2012. Plant intentionality and the phenomenological framework of plant intelligence. Plant signaling & behavior, 7(11), pp.1365-1372.
Trewavas, A., 2003. Aspects of plant intelligence. Annals of Botany, 92(1), pp.1-20.
Volkov, A.G., Carrell, H., Adesina, T., Markin, V.S. and Jovanov, E., 2008. Plant electrical memory. Plant signaling & behavior, 3(7), pp.490-492.

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